Near room-temperature thermocatalysis: a promising avenue for the degradation of polyethylene using NiCoMnO₄ powders

Abstract

Polyethylene has been widely used around the world but its degradation is always restricted to being light- or microorganism-assisted. Here, a new irradiation-free and near room-temperature thermocatalytic approach for low density polyethylene (LDPE) degradation is developed by employing NiCoMnO₄ as thermocatalyst. LDPE/NiCoMnO₄ composites were firstly manufactured by melt blending or physically mixing and then underwent thermal aging in ovens with temperatures ranging from 30 °C to 50 °C in darkness. NiCoMnO₄ is proven to have outstanding thermocatalytic activity and can lead to distinct thermal degradation of composite films both in air and in water. The optimal NiCoMnO₄ dosage in composite films is just 1 wt% and for this specific catalyst loaded composite film, its decomposition temperature and weight-average molecular weight reduced by 53.43 °C and 79.60% respectively, together with an increase of crystallinity by 5.5% after thermal aging in air at 50 °C for 90 days. In addition, a higher degradation degree could be achieved with a further increase of the thermal aging time.

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1. Introduction

During the past decades, polyethylene has been one of the fastest growing commercial thermoplastic polyolefin materials and has been used in lots of application fields due to its low cost and desirable properties.¹ But the high performance and extensive application of polyethylene also results in large amounts of environmental problems at the end of its useful life. Under natural conditions, polyethylene is very stable and hard to be degraded. Traditional disposal methods of waste plastics mainly include recycling, incineration or burying in landfills, but each of these ways has its limitations and disadvantages.^2,3

In recent years, researchers have tended to exploit new degradable polyethylene materials and a series of products have been developed, such as photodegradable, biodegradable and photo-biodegradable polyethylene composites.^3–7 Among them, photodegradable polyethylene composites are most widely studied because of their high efficiency. Once exposed to appropriate light irradiation, electrons and holes will be generated on the surface of the photocatalyst, which may give rise to a redox reaction and finally result in the decomposition of polymer molecules.^8–10 Nevertheless, this attractive process may thoroughly stop when being conducted in darkness. The biodegradation and photo-biodegradation of polyethylene materials are also very interesting; unfortunately, it is still difficult to overcome the slow biodegradation rate and limited contributing microorganism species,¹¹ as well as the requirement for light irradiation.

NiCoMnO₄ powders, because of their high coefficient of temperature sensitivity, have been utilized widely as ingredients for negative temperature coefficient thermistors.^12,13 When the temperature rises, the charge carrier concentration in NiCoMnO₄ also increases, and there are more electrons or holes hopping between Mn³⁺ and Mn⁴⁺, as well as Co²⁺ and Co³⁺ in the octahedron.^14–17 Meanwhile, if the heat energy is appropriate, electrons may be promoted from the valence band to the conduction band and leave holes in the valence band. Most of all, NiCoMnO₄ has been proven to have great thermocatalytic activity in our previous research.¹⁸

Herein, we report a new and promising low-temperature thermocatalytic approach for the degradation of LDPE films by NiCoMnO₄ that can certainty accelerate the aging process even in darkness. The composite mixing method, catalyst dosage, thermal aging environment, thermal aging temperature and thermal aging time are all discussed. The probable mechanism for thermocatalytic degradation is also proposed.

2. Experimental

2.1 Reagents

Nickel nitrate (Ni(NO₃)₂·6H₂O), manganese nitrate (Mn(NO₃)₂) (50%), cobalt nitrate (Co(NO₃)₂·6H₂O), sodium hydroxide (NaOH) and sodium sulfate (Na₂SO₄) were bought from Chengdu Kelong Chemical Reagent Factory. Oxalic acid (H₂C₂O₄·2H₂O) was bought from Shanghai Chemical Reagent Plant. LDPE was bought from Sinopec Group. All chemicals were of analytical grade. They were purchased and used directly without further purification. All the solutions were prepared in distilled water.

2.2 Synthesis of NiCoMnO₄ powders

NiCoMnO₄ powders were synthesized by the co-precipitation method.¹⁸ The final precursor was calcined in a muffle furnace at 700 °C for 2 h, and subsequently cooled down naturally to room temperature.

2.3 Preparation of LDPE/NiCoMnO₄ composite films

Melt blending: the initial LDPE and the as-prepared NiCoMnO₄ powders were pre-mixed by a high-speed mixer first, and then added into an extruder (Thermo Fisher Scientific Inc., Haake Pheomex OS) to manufacture LDPE/NiCoMnO₄ masterbatch granules with 5 wt% catalyst loading. After that, a certain amount of masterbatches were taken and mixed with pure LDPE to blow LDPE/NiCoMnO₄ composite films with 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt% filler content, respectively. Physical mixing: the initial LDPE and NiCoMnO₄ powders were just mixed by a high-speed mixer, and the catalyst contents in the hybrids were also 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt%. Pure LDPE was used as a contrast sample.

2.4 Characterizations

2.4.1 Characterization of the prepared NiCoMnO₄. A powder X-ray diffraction (XRD) pattern was recorded with X′ Pert PRO (PANalytical B.V.) (40 kV, 40 mA) at 2θ (Cu Kα) 10° to 80°. The Brunauer–Emmett–Teller (BET) specific surface area of the catalyst was determined through nitrogen adsorption at −196 °C (JW-BK112, Beijing JWGB Sci & Tech Co., Ltd.). The analysis of particle size was carried out by dynamic light scattering (DLS) (90 Plus Particle Size Analyzer, Brookhaven Instruments Corporation). The surface morphologies of the catalyst were observed by field emission scanning electron microscopy (FESEM, Zeiss Ultra 55) with an accelerating voltage of 15 kV. The catalyst powders were also examined by transmission electron microscopy (TEM, Zeiss Libra 200 FE) equipped with a field emission gun (FEG) operating at a 200 kV accelerating voltage.

2.4.2 Fourier transform infrared spectroscopy (FT-IR). FT-IR measurements were performed on a Nicolet 6700 instrument (Nicolet instrument Co. USA) in the wavenumber range of 400–4000 cm⁻¹. The extent of oxidation of the composites was determined by measuring the levels of carbonyl absorbance and the carbonyl index (C.I.) was used to express the concentration levels of carbonyl compounds, which is calculated as the peak intensity ratio between the carbonyl peak at 1715 cm⁻¹ and a reference CH₂ scissoring peak at 1463 cm⁻¹.^1,19

2.4.3 Thermal analysis. Thermogravimetric analysis (TGA) was carried out using a thermogravimetric instrument (SDT Q600, TA Instruments, USA) to investigate the thermal degradation of the films. All samples were heated from room temperature up to 600 °C under a nitrogen atmosphere with a heating rate of 10 °C min⁻¹.

The crystallization behavior of the films was detected using differential scanning calorimetry (DSC, Q200, TA Instruments, USA) at a heating rate of 10 °C min⁻¹ in nitrogen. All samples were first heated from room temperature to 160 °C to erase their thermal history, and then cooled to room temperature and immediately reheated to 160 °C. The melting temperature (T_m) was determined from the heating curve, while the crystallization temperature (T_c) was calculated from the cooling curve. The crystallinity content (X_c) of the films was evaluated according to the following equation:

where ΔH_m is the melting enthalpy (from 60–120 °C) of the sample, and ΔH₀ is the melting enthalpy for 100% crystalline polyethylene (293 J g⁻¹).²⁰

2.4.4 Morphology analysis. The surface morphologies of the films were observed by field emission scanning electron microscopy (FESEM, Zeiss Ultra 55) with an accelerating voltage of 15 kV.

2.4.5 XPS analysis. X-ray photoelectron spectra were collected using a Perkin-Elmer PHI 5300 X-ray photoelectron spectrometer with Mg Kα radiation (hν = 1253.6 eV, 14 kV, 250 W). The ratio of elemental oxygen to carbon (O/C) was determined from the low-resolution spectra. The C_1s peak obtained from the high-resolution spectra was resolved into three subpeaks: C1, C2 and C3. C1 represented unoxidized carbon, C2 and C3 represented various oxidized carbons. The oxidized to unoxidized carbon ratio (C_ox/unox) was calculated using the following equation:²¹

2.4.6 GPC analysis. The weight-average molecular weight (M_w) and number-averaged molecular weight (M_n) were examined by a high temperature gel permeation chromatograph (HT-GPC, PL-GPC 220, UK). The analytical flow rate was 1 mL min⁻¹ and the temperature was 160 °C. The calibration of the data was conducted using standard polystyrene samples.

2.4.7 Heat current measurements. The heat current of the catalyst at different temperatures was measured using an electrochemical working station (CHI660E, Shanghai, China) in a conventional Teflon electrochemistry cell with a three-electrode system in 0.5 M Na₂SO₄ electrolyte, with a working electrode (NiCoMnO₄), a platinum plate counter electrode and an Ag/AgCl reference electrode. The working electrode was prepared by coating the mixture of NiCoMnO₄ powders, poly(vinylidene fluoride) and N-methyl-2-pyrrolidone on ITO glass. An electrochemical cell was immersed in an Electro-Thermostatic Water Bath and the adjustment of temperature was achieved by heating up or adding ice. The voltage between the working electrode and counter electrode is 0.7 V.

2.5 Thermal aging experiments

The unaged composite films with a 0 wt%, 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt% NiCoMnO₄ dosage were cut into rectangles with sizes of approximately 10 cm × 10 cm. Then, rectangles were averagely put into six separate beakers and in each one, there were films with seven different catalyst dosages ranging from 0 wt% to 5 wt%. After that, these six beakers were placed into three electrothermal blowing dry ovens with temperatures of 30 °C, 40 °C and 50 °C, respectively. In each oven, one beaker was filled with water and the other was exposed to air (Fig. 1). The thermal aging of the physically mixed hybrids with a different catalyst content was carried out at 50 °C in air. The thermal aging experiments were all conducted in darkness. After aging for a specific time, the films or hybrids were taken out and rinsed selectively with deionized water and then characterized by the corresponding methods.

Fig. 1 Schematic diagram of the experimental configuration for the thermal degradation process.

3. Results and discussion

3.1 Characterization of NiCoMnO₄

3.1.1 XRD analysis. The XRD pattern of the catalyst is shown in Fig. 2. The diffraction peaks at 2θ = 18.5°, 30.5°, 35.9°, 43.7°, 54.2°, 57.8° and 63.5° can be indexed to the (111), (220), (311), (400), (422), (511), and (440) lattice planes, indicating the formation of NiCoMnO₄ with a spinel-type cubic structure. The diffraction peaks at 2θ = 37.2°, 43.3°, 62.9° and 75.4° can be indexed to the (111), (200), (220), and (311) lattice planes, demonstrating the existence of NiO. It is clear that the main component of the catalyst is NiCoMnO₄ and NiO is just an impure phase, which is generated possibly due to the inhomogeneous distribution of Ni, Co and Mn in the powders during the sintering process.²² Considering the XRD diffraction peak area, the relative content of NiCoMnO₄ and NiO in the catalyst powders is calculated to be 97.8 wt% and 2.2 wt%, respectively. Meanwhile, NiO has been proven to have no catalytic activity,¹⁸ so mixing in a very small amount of this may hardly affect the catalytic efficiency. Besides, according to the Debye–Scherrer formula, the average crystallite size of the catalyst is 26.4 nm.

Fig. 2 XRD pattern of NiCoMnO₄ powders calcined at 700 °C for 2 h.

3.1.2 Morphology analysis. The morphology and aggregation state of the nanoparticles are vital since they will influence the specific surface area and the catalytic activity. Fig. 3a shows a typical SEM micrograph of the powders after sintering at 700 °C for 2 h. It is obviously observed that the NiCoMnO₄ catalyst is composed of large numbers of nanoparticles with an average size of less than 50 nm. But most of them are interconnected, resulting in the formation of spherical aggregates.

Fig. 3 SEM (a), TEM (b and c), and HRTEM (d) images of the NiCoMnO₄ powders.

The TEM images of the NiCoMnO₄ powders are shown in Fig. 3b–d. The morphology of the catalyst represented in Fig. 3b is in good consistency with the result of the SEM image, in that almost all of the nanoparticles are contiguous. Fig. 3c suggests that most nanoparticles are smaller than 50 nm and some are even approximately 20 nm. Fig. 3d is the HRTEM image of the catalyst powders. It represents the high degree of crystallinity of these nanoparticles, and the clear lattice fringes illustrate that the interplanar spacing is about 2.49 Å and 4.86 Å, which corresponds to the (311) and (111) plane of the cubic NiCoMnO₄.

3.1.3 Particle size and BET surface area analysis. DLS was performed to evaluate the particle size distribution of the catalyst powders dispersed in aqueous medium and the result is shown in Fig. 4. The mean diameter of NiCoMnO₄ in solution is 230.6 nm with a polydispersity index of 0.179, indicating a relatively narrow size distribution. Nevertheless, the particle size measured by DLS is much larger than that observed by XRD, SEM and TEM. Surface contact and agglomeration, hydrodynamic diameter^23,24 and various forces of interaction in the solution²⁵ may all result in this phenomenon. Meanwhile, for the NiCoMnO₄ catalyst, its specific surface area obtained from BET analysis is 7.23 m² g⁻¹. The main characteristic parameters of NiCoMnO₄ are listed in Table 1.

Fig. 4 The particle size and particle distribution of the NiCoMnO₄ powders.

Table 1 Main characteristic parameters of NiCoMnO₄

Phase	Cubic
Lattice parameters	a = b = c = 8.2607 Å
Crystallite size	26.4 nm
Mean size	230.6 nm
BET surface area	7.23 m^2 g^−1

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3.2 Thermocatalytic degradation of LDPE/NiCoMnO₄ composite films

3.2.1 FT-IR analysis. The changes in the chemical groups of the films were evaluated by FT-IR spectroscopy, which also allowed for quantification of the amount of carbonyl groups. Prior to thermal aging (Fig. 5a), initial pure LDPE films exhibit prominent characteristic absorption bands of long alkyl chains for the asymmetrical stretching vibration of –CH₂– at 2919 cm⁻¹, symmetrical stretching vibration of –CH₂– at 2850 cm⁻¹, scissoring vibration of –CH₂– at 1463 cm⁻¹, C–H symmetrical bending vibration of –CH₃ at 1377 cm⁻¹ and inner rocking vibration of –CH₂– at 719 cm⁻¹. The characteristic bands of unaged LDPE/NiCoMnO₄ composite films are the same as those of initial pure LDPE, indicating that the FT-IR spectra character of the polymeric matrix is not affected by embedding the NiCoMnO₄ particles.^26,27

Fig. 5 FT-IR spectra of unaged films with different catalyst dosage (a), pure films before and after aging at 30 °C, 40 °C and 50 °C in water (b) and in air (c), composite films with different catalyst dosage after aging at 50 °C in water (d) and in air (e), composite films with 1 wt% catalyst before and after aging at 30 °C, 40 °C and 50 °C in water (g) and in air (h); C.I. graph of composite films with different catalyst dosage after aging in water and in air at 50 °C (f), pure films and composite films with 1 wt% catalyst dosage aged in air and in water for different times (i). For (b)–(h), the aging time is 90 days.

For pure LDPE films, they are degraded slightly both in water (Fig. 5b) and in air (Fig. 5c), and only a weak band at 1715 cm⁻¹ assigned to the C=O stretching vibration of carbonyl is observed after thermal aging for 90 days even at 50 °C. A proper amount of NiCoMnO₄ is beneficial for the oxidation of polyethylene and the optimal catalyst dosage in the films is just 1 wt% (Fig. 5d–e), which can be obviously illustrated by Fig. 5f. With the increasing catalyst filled percent in the matrix, the C.I. rises dramatically and reaches the maximum value at 1 wt% dosage, but then decreases with a larger filler content. This may be due to the greater electron–hole recombination rate with a higher catalyst dosage, finally contributing to the decline of the total thermal aging efficiency. In order to study the effect of temperature on thermal degradation, LDPE/NiCoMnO₄ films with 1 wt% catalyst content are also aged at 30 °C and 40 °C. Both in water (Fig. 5g) and in air (Fig. 5h), there is a great divergence among these films in the peak intensity at 1715 cm⁻¹, and the temperature is found to be crucial for thermal degradation, and the higher the aging temperature is, the greater the degradation degree is. In addition, a broad peak at around 3400 cm⁻¹, assigned to –OH stretching of hydroperoxide, emerged after being aged at 50 °C, which is an important intermediate. Fig. 5i displays the trends of the C.I. with thermal aging time. The C.I. of pure LDPE increases almost linearly and slightly, and only a very small increase is observed throughout the aging process. For composite films with 1 wt% catalyst, their C.I. remains relatively low at short exposure times but increases rapidly after aging for 60 days, finally reaching up to 0.74 in air and 0.47 in water after 90 days. Besides, the degradation degree of the films is much more distinct in air than in water. Oxygen must be the major factor because it is the essence of the formation of superoxide (O₂˙⁻) and hydroxyl radicals (˙OH) and both of these are highly reactive species. Oxygen is also indispensable for the further oxidation of intermediates produced during the degradation of the films.^2,18 Moreover, the controlled experiment by physically mixing NiCoMnO₄ with LDPE powders was carried out and in this case, the optimal catalyst dosage in the hybrids is 3 wt% and the C.I. reaches 0.43 after thermal aging in air at 50 °C for 90 days (Fig. S1†), despite the relatively large distance and tight contact between the catalyst and LDPE. This demonstrates that NiCoMnO₄ can definitely accelerate the degradation of LDPE.

3.2.2 TG analysis. TG curves of pure LDPE films and LDPE/NiCoMnO₄ composite films with 1 wt% catalyst before and after aging at 50 °C for 90 days are shown in Fig. 6. The characterizations of thermal decomposition for each film are expressed in terms of the initial decomposition temperature (T_0.5%, defined as 0.5 wt% mass loss), the decomposition temperature (T_5%, defined as 5 wt% mass loss), and the peak temperature (T_P) from the derivative thermogravimetric curve.

Fig. 6 TG curves of pure LDPE films (a), and composite films with 1 wt% catalyst (b), before and after aging at 50 °C for 90 days.

For pure LDPE films (Fig. 6a), the unaged samples present a relatively good thermal stability since no significant mass loss (<0.5 wt%) occurred until 400.05 °C. But after thermal aging in air, T_0.5%, T_5% and T_P reduced by 45.92 °C, 22.76 °C and 7.25 °C, respectively, conveying that a distinct thermal degradation has taken place. In contrast, pure LDPE films aged in water degraded slightly.

In comparison to initial pure LDPE films, T_0.5%, T_5% and T_P of the unaged LDPE/NiCoMnO₄ composite films with 1 wt% catalyst are all relatively low (Table 2), which may be due to a slight thermal degradation of LDPE caused by NiCoMnO₄ powders which happened during film processing. Towards the composite films, T_P remains almost the same, but T_0.5% is dramatically advanced by 190 °C in air and 111 °C in water. T_5% also declined by 53.43 °C in air and 9.3 °C in water, indicating the generation of unstable substances in the ageing process and the difference on the aging degree of water-aged and air-aged films should be the different oxygen levels.

Table 2 TGA data of pure LDPE films and LDPE/NiCoMnO₄ composite films with 1 wt% catalyst before and after degradation at 50 °C for 90 days

Samples	T0.5% (°C)	T5% (°C)	TP (°C)
N-0% initial	400.05	445.04	488.94
WN-0% 50 °C	388.38	443.66	485.72
AN-0% 50 °C	354.13	422.28	481.69
N-1% initial	386.92	437.48	485.25
WN-1% 50 °C	264.86	428.18	485.68
AN-1% 50 °C	186.04	384.05	484.91

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3.2.3 DSC analysis. The crystallization behavior of the films is displayed in Fig. 7 and characterization data are listed in Table 3. The filler content of NiCoMnO₄ in the composite films is 1 wt% and the films are aged at 50 °C for 90 days.

Fig. 7 DSC spectra for the pure films and composite films with 1 wt% catalyst before and after aging for 90 days in air and water at 50 °C: the melting (a) and crystallization (b) curves of the pure films, and the melting (c) and crystallization (d) curves of the composite films.

Table 3 DSC data of the pure LDPE films and LDPE/NiCoMnO₄ composite films with 1 wt% catalyst before and after degradation at 50 °C for 90 days

Samples	Tm (°C)	Tc (°C)	Xc (%)
N-0% initial	111.21	98.62	39.7
WN-0% 50 °C	111.23	98.71	41.0
AN-0% 50 °C	112.45	99.47	41.6
N-1% initial	111.86	98.68	40.2
WN-1% 50 °C	113.04	104.04	43.2
AN-1% 50 °C	113.52	105.41	45.7

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Fig. 7a shows a sharp endothermic peak at 111.21 °C, which indicates T_m of the unaged pure LDPE. After thermal aging in air, T_m of the pure films shifts significantly, together with an increase of X_c by 1.9%, while T_m and X_c of the water-aged films change less prominently. The cooling scans of the pure LDPE films are presented in Fig. 7b. After thermal aging, no matter in water or in air, T_c of the pure films increases slightly, which is consistent with the change in X_c.

Fig. 7c and d show the heating and cooling curves of the LDPE/NiCoMnO₄ composite films. T_m, T_c and X_c of the unaged composite films are all a little higher than those of the unaged pure films, which is also a result of thermal degradation induced by NiCoMnO₄ during processing. The catalyst in the films can also act as a heterogeneous nucleating agent and affect the melt and crystallization behavior of the films as well. After thermal aging, endothermic curves are visibly lower in the range from about 95 °C to 113 °C (note the arrows in Fig. 7c), suggesting that the generation of intermediate substances during thermal degradation and that are less thermally stable.²⁸ And as always, the aging phenomenon is more pronounced when being conducted in air. It is worth noting that the corresponding T_m, X_c and T_c of the air-aged composite films increase by 1.66 °C, 5.5% and 6.73 °C, respectively (Table 3). The thermal degradation behavior of the films should mainly be attributed to abundant reactive species generated on the surface of NiCoMnO₄ under heat excitation and that can attack polyethylene molecules decisively. Once the breaking of the molecular chain happens in the amorphous region, low molecular weight segments will generate and then crystallize or act as nucleating agents for enhancing the rate of crystallization. Chain scission also gives rise to sufficient chain mobility to produce secondary crystallization.²⁹ The formation of the carbonyl group may lead to the increase of X_c as well.³⁰

3.2.4 Microscopy investigation. As shown in Fig. 8, scanning electron microscopy was carried out to observe the surface morphology changes of the films. Fig. 8a and d represent the pure LDPE and LDPE/NiCoMnO₄ composite films with 1 wt% catalyst before aging. The surface of these films is uniform without any pores and NiCoMnO₄ particles are almost not visible as they are buried inside the composite films. After thermal aging, small cavities are observed on the surface of the pure LDPE films, and the degradation is a little more distinct in air (Fig. 8c) than in water (Fig. 8b). For the composite films, the surface is destroyed seriously, especially those aged in air (Fig. 8f), suggesting that the NiCoMnO₄ catalyst has greatly enhanced the degradation of the LDPE, and the formation of these irregularities on their surfaces is induced by the escape of volatile products from the LDPE matrix.⁹

Fig. 8 SEM micrographs of pure films before (a) and after aging in water (b) and in air (c); composite films with 1 wt% catalyst before (d) and after aging in water (e) and in air (f). The aging is conducted at 50 °C for 90 days.

3.2.5 Weight change analysis. Fig. 9 illustrates the weight change of the films during thermal decomposition. Yet despite low molecular weight compounds being produced after thermal degradation and some volatile products escaping from the LDPE matrix, the weight of the aged films has increased to some extent, especially for the composite films with 1 wt% catalyst aged in air. This can be attributed to the incorporation of oxygen into the polymer chain as a consequence of oxidation and the rate of oxygen incorporation is higher than that of the volatiles escaping.³¹ Meanwhile, the trend of weight change is in agreement with that of the C.I. (Fig. 5i).

Fig. 9 Weight change of the pure and composite films with 1 wt% catalyst dosage in water and in air for different aging times.

3.2.6 XPS analysis. To accurately determine the types and amounts of carbon–oxygen bonds present, the surface of the films was investigated by XPS and the spectra of C_1s and O_1s are shown in Fig. 10. The values of C_ox/C_unox and O/C for the different samples are also given in Table 4. The C_1s peak is resolved into three component peaks at binding energies of 284.6 eV (C1), 286.2 eV (C2) and 288.0 eV (C3), assigned to C–C or C–H, C–OH and C=O, respectively.^21,32,33 For the unaged pure films (Fig. 10a), except for the unoxidized carbon (C1), a weak subpeak (C2) corresponding to C–OH is also observed, which may be due to a slight oxidation occurring on the surface during the processing. After thermal aging, a C3 peak appears in both the pure films (Fig. 10b) and composite films with 1 wt% catalyst (Fig. 10c), indicating the formation of carbonyl bonds. Meanwhile, a drop in the C1 peak is observed as well, which is attributed to a decrease in the concentration of unoxidized carbon, especially for the composite films, and this phenomenon can be quantitatively proven by the value of C_ox/C_unox in Table 4. Fig. 10d shows the O_1s spectra of the LDPE films. The O_1s peak (532 eV) of the aged films increased markedly, demonstrating the introduction of oxygen into the LDPE chains which is in agreement with the value of O/C.

Fig. 10 C_1s and O_1s XPS spectra of the LDPE films before and after thermal aging at 50 °C in air for 90 days.

Table 4 XPS data of the LDPE films before and after degradation at 50 °C for 90 days

Samples	Cox/Cunox	O/C
N-0% initial	0.07	0.06
AN-0% 50 °C	0.09	0.09
AN-1% 50 °C	0.14	0.13

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3.2.7 Evaluation of molecular weight changes. Fig. 11 shows the molecular weight distributions of the films before and after aging at 50 °C for 90 days in air. It is clear that the average molecular weight and distribution of the films change evidently after thermal degradation, especially those mixed with the NiCoMnO₄ catalyst.

Fig. 11 Molecular weight distribution curves of unaged LDPE, aged LDPE and LDPE/NiCoMnO₄ films at 50 °C in air for 90 days.

The M_w and M_n values of the unaged pure films are 31 583 g mol⁻¹ and 3726 g mol⁻¹. After thermal aging, M_w and M_n of the composite films reduced by 79.60% and 18.14%, respectively, while M_w and M_n of the pure LDPE film decreased only by 23.15% and 3.38%. The decline in M_w and M_n is due to chain scission reaction in the polymer matrix during thermal oxidation. The GPC results suggest that NiCoMnO₄ plays an important role in the thermal degradation of LDPE and has accelerated the decomposition greatly.

3.3 Mechanism of the thermodegradation process

Fig. 12 shows the probable schematic of thermocatalytic degradation of polyethylene. In NiCoMnO₄, Ni ions prefer the occupation of tetrahedral sites, and Co ions prefer the occupation of octahedral sites, while Mn ions partially occupy the tetrahedral and octahedral sites.^15,34 Upon exposure to a specified temperature, electrons may be excited to the conduction band, leaving holes in the valence band (eqn (1)). Electrons or holes may hop between Mn³⁺ and Mn⁴⁺, as well as Co²⁺ and Co³⁺ at appropriate temperatures (eqn (2) and (3)).^14–17 The change in carrier concentration of NiCoMnO₄ at different temperatures can be experimentally proven by heat current measurement (Fig. 13) since that can indirectly reflect the carrier concentration level in the catalyst. The lowest temperature is set at 20 °C as thermodegradation almost didn’t happen at this temperature (Fig. S2†). For each specified temperature, the heat current increases continually and achieves the maximum as the catalyst temperature reaches the set value, then decreases rapidly with the further addition of ice, suggesting that NiCoMnO₄ is very sensitive to temperature changes. Besides, the heat current of the catalyst at 50 °C is significantly larger than that at 30 and 40 °C, which means a higher thermocatalytic activity at 50 °C and this is in line with all the above analyses. These carriers may move to the surface of the catalyst and react with adsorbed oxygen molecules and water to produce superoxide anions (O₂˙⁻) and hydroxyl radicals (˙OH) (eqn (4)–(9)), of which ˙OH is the most important oxidant in catalytic oxidation.⁹

Fig. 12 Schematic of thermocatalytic degradation of LDPE by the NiCoMnO₄ catalyst.

Fig. 13 Heat current curve of NiCoMnO₄ at different temperatures.

In solid-state photocatalytic degradation, hydroperoxide is commonly the major oxidative degradation product of PE and is also a potentially powerful initiator of the carbonyl group, which is the precursor of Norrish-type reactions.^29,35 In our research, hydroperoxide and carbonyl groups are also obviously detected in the composites, suggesting that NiCoMnO₄ has undoubtedly accelerated the oxidation of LDPE and the further thermocatalytic degradation process may also follow Norrish-type reactions just like photocatalytic degradation. The distinction between these should mainly be present in the carrier generation pathway, that the carriers in our research are produced by low-temperature heat excitation. Then the probable degradation process can be briefly described as follows: reactive oxygen species spatially extend into the polymer matrix and attack neighboring LDPE polymer chains to form carbon-centered radicals such as –CHCH₂– (eqn (10)). These are then introduced to the polymer chains and give rise to the cleavage of polymer chains with oxygen incorporation and produce species containing hydroperoxide and carbonyl groups. These intermediates are further catalytically oxidized to carbon dioxide and water (eqn (11)). After degrading LDPE, the NiCoMnO₄ powders were characterized again by XRD and were found to be stable since no change is observed both in the structure (Fig. S3†) and weight. While the degradation of pure LDPE at specified temperatures is straightforward, this happened via direct absorption of heat energy by macromolecular LDPE which then undergoes oxidation reactions.

NiCoMnO₄ + heat → NiCoMnO₄(e⁻ + h⁺) (1)

e⁻ + O₂ → O₂˙⁻ (4)

O₂˙⁻ + H₂O → HO₂˙ + OH⁻ (5)

2HO₂˙ → H₂O₂ + O₂ (6)

OH⁻ + h⁺ → ˙OH (8)

H₂O_ads + h⁺ → ˙OH + H⁺ (9)

–CH₂CH₂– + ˙OH → –˙CHCH₂– + H₂O (10)

4. Conclusions

In the absence of light irradiation, the near room-temperature thermodegradation of LDPE is successfully achieved both in air and in water by NiCoMnO₄ and the degradation process is considerably obvious. Though the initial carriers in this research are excited by heat, the thermocatalytic degradation may also follow Norrish-type reactions just like photocatalysis since hydroperoxide and carbonyl groups are both detected. After aging at optimum conditions for 90 days, M_w of the composite films reduced dramatically by 79.60% and the degradation rate is distinctly faster than that in previous research in oil or under natural sunlight irradiation.^36,37 The degradation degree of the physically mixed hybrids is also evident. And the performance of NiCoMnO₄ could be further improved in many ways, such as optimizing the catalyst synthesis method³⁸ (increasing BET surface area), coating the catalyst with graphene^39,40 (reducing charge recombination) and working synergistically with other catalysts (synergistic effect) (Fig. S4†). This work may open a new avenue for the degradation of polymer wastes, especially those buried in the ground, located in darkness, or even in deep water. It also predicts that some other thermosensitive materials may be useful catalysts as well, such as transition metal oxides⁴¹ and transition metal sulfides (Fig. S5†).

Acknowledgements

This work was supported by The National Key Technology R&D Program, Minister of Science and Technology, People’s Republic of China (Grant No. 2007BAE42B04) and Postgraduate Innovation Fund Project by Southwest University of Science and Technology (Grant No. 13ycjj12).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16626h

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